1. Field of the Invention
The present invention relates to circuitry for providing in a power supply a signal representative of the current flowing in the power switching devices of the supply to thereby control the operation of the supply and more particularly to enhanced versions of that circuitry which allow a number of desirable effects such as increased closed loop a-c gain, increased rejection of common mode noise and accurate representation of low current levels to occur.
2. Description of the Prior Art
Telephone central offices use d-c voltages for a variety of functions. These d-c voltages may be supplied either directly from a bank of batteries or from a power supply which converts the commercially available 60 Hz voltage from the electrical utility into a d-c voltage of proper amplitude. Even if the d-c voltage is supplied directly by batteries, a power supply is ordinarily used to convert the commercial a-c voltage into a d-c voltage for maintaining the charge on the batteries. Where the supply is used for the purposes of charging the batteries after a discharge and/or supplying office load it will be referred to hereinafter as a "charger" and where the supply is used in place of the batteries it will be referred to hereinafter as an "eliminator".
Whether the power supply is used at the central office as a charger or an eliminator, it provides the d-c voltage at its output from the commercially available a-c voltage at its input. Sometimes, and especially when the operating frequency of the supply is above the frequency of the commercial a-c voltage, the supply rectifies the commercially available a-c voltage to peak charge a bank of capacitors at its input. As a result of this rectification, a d-c voltage having a large ripple appears on the capacitors. This ripple has a principal frequency which is 120 Hz or twice the 60 Hz frequency of the commercially available a-c voltage. Ordinarily the amplitude of this ripple on the capacitor bank is much greater than that which is allowed to be present at the output of the charger or eliminator. In addition, as the charger or eliminator is often used to provide d-c voltage for the subscribers' telephones connected to the central office, this ripple may appear on the telephone lines in the form of audible and therefore objectionable noise. It is therefore desirable that the charger or eliminator function in a manner so as to attenuate the ripple.
In the past it was common for chargers and eliminators to be designed so that their power circuitry and control circuitry operated at the 60 Hz frequency of the commercially available a-c voltage. In this manner the ripple could be attenuated through the use of various components designed to operate at that low frequency or harmonics thereof. In order to reduce the size, cost and objectionable noise that ordinarily are associated with chargers and eliminators operating at 60 Hz it has now become common for such supplies to operate at a frequency above the audible range. Selection of such an operating frequency has the desirable characteristic of reducing the size of the supply and any transformers therein which results in a savings of both component costs and supply weight and, in addition, the generation of audible noise by the circuitry used in the supply. The selection of such a high operating frequency does not, however, cause the attenuation of the objectionable ripple described above. One way in which that ripple may be attenuated is to use a filter either at the input or the output of the supply. Such a filter would have to operate at the relatively low frequency of the ripple and would, therefore, involve the use of components which would negate any cost savings obtained by having the supply operate at the higher than audible frequency. It is therefore desirable to find another means by which the ripple may be attenuated without introducing extra cost back into the supply.
Supplies which generate a d-c voltage at their output normally operate in a mode so as to maintain regulation of their voltage. Such regulation is obtained by feeding back a sample of the output voltage to the control circuitry of the supply to thereby control the operation of the power devices in the supply. The voltage control or regulation circuitry associated with the charger or eliminator therefore is designed to have a frequency response which attenuates the objectionable ripple. Power supplies are also designed to operate in a mode wherein instead of maintaining the voltage at their output regulated they limit the current flowing through the power devices in the supply. A supply which operates in that mode will be referred to hereinafter as being in its "current limited mode" of operation.
Power supplies do not typically operate in their current limited mode but do so usually only momentarily and only as a result of an overload condition at their output. Chargers and eliminators, on the other hand, are ordinarily operated in parallel without a measure of true load sharing between the supplies. For that condition of operation at least one of the chargers or eliminators in the parallel combination ordinarily operates in its current limited mode. Control of current is effected in a manner similar to the regulation of the voltage described above. The charger or eliminator includes a circuit in its current control loop which takes a sample of the alternating current flowing through the power devices and derives therefrom a d-c signal representative of that current. The control loop uses that d-c signal to maintain the current in the power devices at some predetermined value. It is therefore desirable that this current control loop also function to attenuate the undesirable ripple. It is also desirable that this current control circuit respond relatively quickly to any sudden change in the current to thereby protect the power devices of the supplies.
Chargers and eliminators may also use the d-c signal representative of the current flowing in the power devices to not only control the amplitude of that current but also in circuitry which responds to the falling of that current below some predetermined minimum amplitude. This circuitry may then either control the operation of the supply or provide a signal indicative of the fact that the current has fallen below that minimum amplitude. As an example, when the supply is used as a charger it might include a circuit which gives rise to an audible and/or visible alarm when the current falls below the minimum amplitude. The occurrence of this alarm indicates that the current supplied by the charger has fallen below the minimum level needed at the central office to supply the office load and/or recharge the batteries after they have been discharged.
In any case, the predetermined minimum amplitude of power device current is usually only some relatively small percentage of the current flowing in the devices at full load. It is therefore desirable that this d-c signal representative of power device current be a true representation of that current particularly at such low amplitudes. In this manner an accurate determination can then be made as to when the current falls below the minimum predetermined amplitude.
Chargers and eliminators, as is well known in the art, may be embodied by using either a single power device or an even number of power devices. The single device conducts only for a part of the period of the operating frequency of the supply and is nonconductive for the remainder thereof, whereas the even number of devices conduct alternately. In those supplies embodied by a single device, the circuitry for deriving a signal representative of the current flowing in the device is relatively insensitive to common mode noise. When the supply is embodied by an even number of such devices, the above-mentioned circuitry, whether or not it includes the desirable high a-c gain and/or the desirable response to low current amplitude, is sensitive to such noise. It is, therefore, desirable that this circuitry for those supplies having an even number of power devices also be insensitive and, therefore, be unresponsive to common mode noise.
As will be described in more detail hereinafter with respect to FIGS. 2a and 2b, while the current control circuits of the prior art do have the desirable speed of response, they do not have the desirable ripple attenuation characteristics or the desirable response to low amplitude currents. In addition, as will be described in more detail hereinafter with respect to FIG. 2b, the prior art current control circuit for a supply having an even number of power devices is sensitive to the common mode noise. The circuit of the present invention represents an enhancement over those prior art current control circuits in that it not only has the desirable speed of response but also may have any one or more of the desirable features described above.